12AU7 Overdrive Pedal

[PRR]

> anode loading selection

For what? Maximum gain? Maximum output? Minimum THD? MAXimum THD?

Radiotron Handbook 3rd ed. Pick plate resistor 2X to 5X the tube plate resistance. Trial cathode resistor Mu times smaller than plate load resistor. That'll work.

At least for 100V-300V operation. Plate resistance goes screwy at low voltage. And in some sense "maximum output" may be critical because there is SO little output possible at low supply voltage. In fact it may be hard to get ANY output.

You really gotta set up a tube on a breadboard with a resistor assortment and try it.

[Scootsit]

I appreciate it. I ordered everything yesterday, I ordered a few different resistors, and have a hundred more in my parts drawer, I'll play a bit. What you mentioned is what I was struggling with, the tubes will inherently behave differently than expected at 24v. Anyway, I'll have the parts next week and way too much free time, so I'll post what I can. I also think I have a trim pot or two laying around, so I may use that.

[tubegeek]

Here is an excellent tutorial by Merlin "Valvewizard" Blencowe on small-signal triode amplifier stage design which will explain the process in greater detail:

http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

The examples you will see there are assuming higher supply voltages, though.

The characteristic curves for extremely low voltages are there on the data sheet but there are some problematic aspects to them - the curves are very crowded and small in that area, and the tubes themselves will vary quite a bit from the published curves at the extremes. Hell, they vary a fair amount even in their typical operating ranges. However, "close enough for audio" is a pretty loose standard (thank God.)

Google "starved plate triode" for a variety of useful links.